This invention relates generally to threaded fasteners, and in particular to tamper-resistant threaded fasteners.
It is well known in the art to provide nuts, bolts and other fasteners with various anti-tampering features designed to make it difficult for unauthorized persons to remove a part, gain access to a piece of equipment, or otherwise disassemble an object where such fasteners have been employed. A first variety of anti-tamper fasteners have a drive portion that sheers off when a certain torque is reached, leaving a smooth surface that is difficult to remove. The disadvantage of these anti-tamper fasteners, however, is that authorized persons cannot remove them without resorting to crude methods that inevitably leave the fastener mutilated. A second variety of anti-tamper fasteners are designed to resist removal except when a driver specifically designed for the fastener is used (e.g. by an authorized person). U.S. Pat. No. 4,938,108 issued to Mekler provides a relatively comprehensive background summarizing various of these type of anti-tamper fastener designs.
A popular method of providing an anti-tamper feature for a fastener used with a special driver is to design the fastener to have a recessed drive socket (hex, TORX®, Bristol, Phillips, etc.) with a center pin or post designed to prevent insertion of a screwdriver, knife, or other tool that does not exactly match the profile of the recessed drive/post combination. The TORX® TR is a popular version of center-post anti-tamper fasteners.
A disadvantage of center-post type anti-tamper fasteners, however, is that because the center post is cylindrical, a determined thief can fabricate a tool for removing the anti-tamper fastener simply by drilling a hole through the center of a conventional hex, TORX®, Bristol, or Phillips driver thereby defeating the anti-tamper feature. Another disadvantage is the center post, being cylindrical, does not transmit any torque when using an authorized drive tool and, therefore, the recess must carry the entire load.